Optical tracking is a general purpose tool that can be used as an enabling technology in a broad range of applications. One example is determining the axial direction of a laser beam. For example, a suitably accurate optical tracker could be incorporated into a surface scanning device which, by sending out a laser beam towards a surface with unknown orientation and measuring the reflected beam's axial direction, determines the surface orientation with respect to the scanning device. Measuring multiple surface orientations in this manner also provides information about the relative orientation of these multiple surfaces or surface segments, and thus generates a contour map.
Optical tracking devices can also be used aboard stationary or moving platforms to determine their position or orientation with respect to one or more light sources. One example application is a ground based observation station which is tasked with tracking spacecraft in Earth orbit. An optical tracking device is required to measure and track the axial direction of light coming from one or more such spacecraft. This light can be reflected sunlight, reflected laser light, or light originating from the spacecraft itself. A second example is an airborne reconnaissance platform tasked with tracking moving ground vehicles. The light reflected from or originating from the ground vehicle can be registered by an optical tracker mounted on the airborne platform. The multiple image frames can further be processed to determine the trajectory of the ground vehicle. A third example is an optical stellar tracking device, commonly known in the art as a star sensor or a star tracker. A star tracker determines the orientation of the spacecraft it is mounted on with respect to a known star field or star constellation. The accuracy with which the tasks outlined in these three examples are performed depends, at least in part, upon the accuracy of the optical tracking device used.
An improved optical tracking device and method of tracking the position of a light source have been disclosed in U.S. Pat. No. 8,045,178. The device disclosed is essentially a single axis tracking device because it uses one pair of gratings to shear the incoming light in one direction and is capable of improving tracking accuracy along that direction only. Along the axis that is perpendicular to the shearing direction, this single axis tracking device will have accuracy no greater than that of the coarse position estimate provided by the location of the image of the light source on the image plane. To increase tracking accuracy along two axes, the apparatus disclosed in U.S. Pat. No. 8,045,178 prescribes that two interferometric tracking devices be coupled together. The coupled arrangement could involve using two or more of the single axis interferometric tracking devices with their shearing axes oriented to coincide with the axes along which higher accuracy is desired. This would at least double the size and weight of the overall device. Using a single two-axis tracking device is more practical or desirable than using two single axis tracking devices for many applications. Such a two-axis tracking device is disclosed in U.S. Pat. No. 9,297,880, and even though it represents a significant improvement in both size and weight, improvements to the tracking accuracy are desirable.